I'm 20 miles up Charlotte Harbor and the Peace River from Boca Grande Pass (tidal area). Tarpon can gulp air, salinity isn't necessarily an issue for them. They are occasionally found in land locked fresh water ponds around Punta Gorda. Small ones get there during the rainy season via drainage ditches then get locked in and grow. A lot of the smaller and bigger ones stay here in the river and area canals rather than migrate in the winter.
In the 80s I read an article in the Lauderdale Sun about a Tarpon they found in a swimming pool of an abandoned house in Ft Lauderdale or Miami. The one to the left was caught 8 miles further up river. Bull Sharks get way up too, we catch them Tarpon fishing and I had a friend have a 100+ pound Tarpon bit in half way, way up river.
By the way, UM had something called the Tarpon Bonefish Research Center a while back, not sure if it's still active. Anyone know?